1. Technical Field
This invention relates to rubber compositions particularly for use in the manufacture of automobile tires of a steel-belted configuration.
2. Prior Art
To cope with strict quality requirements for automobile tires, steel-belted tires have been widely used for their good driving stability, high abrasive resistance and other superior physical properties compared to conventional tires. A key determinant of the quality of steel-belted tires is the adhesion between steel cords and rubber coats. This parameter of adhesion dominates the service life of the tire exposed to various environmental conditions during and subsequent to its processing stage and delivery to the user.
To manufacture a steel-belted tire, the starting rubber should have sufficient "initial adhesion" to the steel cords at the time of vulcanization. Unvulcanized rubber, however, tends to absorb moisture and hence decline in its adhesive property, namely "adhesion in water before vulcanization". Vulcanized rubber is also liable to suffer a reduction in its adhesive property, namely "adhesion in water after vulcanization", due to penetration of moisture in the air through the rubber coat into the steel cords during use of the resulting tire. Moreover, if the tire gets cuts or nail punctures in its tread portion during running, water will penetrate into the tire through those damaged portions. This water is warmed up by the heat generated in the running tire and flows through the interstices of the steel cords, causing the rubber coat to separate due to a loss of its adhesive property, namely "adhesion in warm water after vulcanization".
Various rubber compositions are known which are developed to meet some, but not all, of the above adhesion requirements.
The vulcanization process in the manufacture of steel-belted tires is mainly responsible for productivity. Vulcanization may be effected for a shorter period of time but at a higher temperature. However, conventional rubber compositions containing cobalt salts of organic acids will in most cases show a sharp decline, because of the higher vulcanization temperature, in their "adhesion in water before vulcanization", "adhesion in water after vulcanization" and "adhesion in, warm water after vulcanization". Those cobalt salts are commonly accepted as conducive to adhesion to steel cords in the rubber art.
In general, rubbers of high hardness and high modulus are chosen to cover hard steel cords for steel-belted tires. Such hard rubbers often incorporate large quantities of sulfur and carbon black and exhibit high hardness and modulus properties upon vulcanization. At the same time, however, they are susceptible to insufficient extension on break and objectionable separation at a belt end portion, namely breakage of the rubber coat between belt layers.